Fighters from the Islamic State of Iraq and the Levant (ISIL) have planted landmines and explosives in the ancient Syrian city of Palmyra, which they captured last month, sources told Al Jazeera.

Activist and Palmyra resident Nasser al-Nasser said that ISIL fighters have assembled explosives around several heritage sites in the city.

"We have seen them put the explosives around several sites, we all fear they might blow these ruins up. We can confirm that two sites have been mined. We think ISIL is planning a retaliation following all those air strikes," Nasser said.

"The government has stepped up its air strikes on the city, in the past three days they conducted almost 75 air strikes. The past few days have been horrible," he added.

Activists from the British-based Syrian Observatory for Human Rights also said on Sunday that it was not clear whether the group planned to use the bombs to destroy the UNESCO world heritage site or stop government forces from trying to advance.

The Observatory, which relies on a network of sources on the ground, said the explosives were laid on Saturday.

Fears mount over fate of Syria's ancient Palmyra ruins

"But it is not known if the purpose is to blow up the ruins or to prevent regime forces from advancing into the town, it seems clear that the Syrian regime has increased its air strikes and wants to push ISIL out " said Observatory Director Rami Abdel Rahman.

He said regime forces had launched heavy air strikes against the residential part of the strategic city located in Homs province in the past three days, killing at least 11 people.

"The regime forces are to the west outside the city, and in recent days they have brought in reinforcements suggesting they may be planning an operation to retake Palmyra," he added.

ISIL took control of the city, which is famed for its extensive and well-preserved Greco-Roman ruins, on May 21 after the Syrian government forces retreated.

Outstanding universal value

The group has regularly heavily mined its territory to make it more difficult to recapture.

The city's fall prompted international concern about the fate of the heritage site described by UNESCO as of "outstanding universal value".

Before it was overrun, the head of the UN cultural body urged that the ruins be spared, saying they were "an irreplaceable treasure for the Syrian people, and the world".

ISIL has released several videos in the past documenting its destruction of heritage sites in Iraq and Syria where it controls vast territories.

There have been no reports of damage to sites in Palmyra since the group seized it, though the group's fighters reportedly entered the city's museum, which had largely been emptied of its collection before the fighters arrived.

The group executed more than 200 people in and around Palmyra in the days after capturing the city, including 20 who were shot dead in the ancient ruins, according to the Observatory.

Before Syria's war began, more than 150,000 tourists visited Palmyra each year, admiring its beautiful statues, more than 1,000 columns and formidable necropolis of over 500 tombs.